Resistive feedback amplifiers are often used in the implementation amplifiers, such as Low Noise Amplifiers (LNAs). Several concerns associated with the design of amplifiers utilized for low noise radio frequency (RF) amplification include: (1) providing a sufficiently low Noise Figure (NF); (2) achieving high gain; and (3) the realization of minimum power consumption.
FIG. 1 illustrates a conventional resistive feedback amplifier 100 that may be used in many wireless devices. Such wireless devices may require the amplifier 100 to achieve a low NF, provide reasonable gain with sufficient linearity, a predetermined input impedance and low power consumption. Unfortunately, the amplifier 100 has difficulty realizing these requirements due to the fairly broad bandwidth requirements of many current wireless devices. To demonstrate, consider the following.
In general, the resistive feedback amplifier 100 illustrated in FIG. 1 has an input resistance that is given by
      R    input    =                    R        F            +              R        L                    1      +                        g          m                ⁢                  R          L                    where gm is the transconductance gain of the NPN transistor, RF is the feedback resistor and RL is the load resistor. The gain of the amplifier 100 is given by
      A    v    =            -              (                              g            m                    -                      1                          R              F                                      )                    (                        1                      R            F                          +                  1                      R            L                              )      
A higher transconductance gain may create a larger loop gain, which leads to better linearity. Moreover, a higher transconductance gain will generally increase current consumption of the amplifier 100. However, for higher frequency operation, which is generally needed for broad bandwidth technologies, this increased current consumption is normally required to obtain enough gain.
With the resistive feedback amplifier 100 illustrated in FIG. 1 it is generally difficult to obtain both high gain and large bandwidth. This is because there is a tradeoff between the gain and the bandwidth. In addition, due to the limited bandwidth of the amplifier 100, the input impedance tends to behave inductively rather than resistively. This may cause the gain to plateau or even decrease at high frequencies. In some cases, the input impedance may be capacitive, because the input impedance of the input transistor is capacitive.